1. Field of the Invention
The present invention relates to a fuel injection system for self-ignition internal combustion engines, and includes an injection pump that delivers metered fuel quantities, at periodic intervals, via a connecting line to an injection valve. At least part of the connecting line comprises two different-length circuits which are connected in parallel and which cause the fuel quantity supplied via a feed line to be divided in such a manner that, on the one hand, a small fuel quantity (pre-injection quantity) passes through the shorter circuit and, on the other hand, a larger fuel quantity (main injection quantity) passes through the longer circuit (time-delay circuit), with the two fuel quantities reaching the injection valve with a time lag relative to one another.
2. Description of the Prior Art
Improving fuel consumption is one of the most pressing objectives of development, even in the case of the Diesel engine, which is inherently economical. Faster injection as well as finer and larger-volume atomization of the fuel are usual means of achieving the shortening of the combustion period desired in this connection in order to increase the efficiency of the Diesel cycle in this manner.
The well-known disadvantage of these measures is the high rate of increase of the combustion chamber pressure subsequent to ignition. This is accompanied by the excitation, by higher energy gas forces, of natural modes of the engine structure, which explains the undesirably high air-borne noise emission of such engines optimized for fuel economy. In the face of anticipated further tighter legislation with respect to noise control in internal combustion engines, the designer is confronted with the task of finding means and measures to lower the rate of pressure rise in the combustion chamber--which is a function of the processes taking place during ignition of the mixture--without adversely affecting fuel economy.
A promising concept in this connection has proved to be pre-injection. This involves subdividing fuel injection into two separate timed phases. The first phase provides for a small amount of fuel to be injected, and is followed at a defined time interval by a larger fuel volume (main injection) in the second phase.
In this concept, it is important that it should be possible to make defined settings of both the amount of pre-injected fuel and the time interval between pre-injection and main injection. A high degree of reproducibility of both parameters, which includes their independence of operational state parameters of the engine, such as torque and speed, but also injection line pressure, is another requirement which the modified injection system should satisfy.
The size of the fuel volume for pre-injection is important in as much as it determines the amount of radicals released during the so-called cold-flame and blue-flame pre-reaction phase (of the pre-injected fuel). The latter is decisive for the degree to which the ignition lag which characterizes the main injection is shortened. Since a short ignition lag (which is a prerequisite for lowering the rate of pressure rise in the combustion chamber) is the objective, it is therefore necessary to provide a minimum metered feed of pre-injection fuel to enable sufficient free radicals to be available to bring about the above-mentioned desirable reduction in ignition lag.
The defined setting of a constant time interval between pre-injection and main injection as stipulated above also serves to maximize the amount of radicals that are available. This is because the longer the cold-flame and blue-flame pre-reaction triggered by pre-injection is left alone (e.g. until briefly before the commencement of the hot explosion flame), the greater the enrichment of the fuel-air mixture with radicals.
A pre-condition for optimizing the time interval to be set between pre-injection and main injection is that the engine-specific ignition lag should be only slightly dependent on the operationally dictated load and speed variations of the engine.
Apart from cold starting or the transition from an extended low-load phase to full load (turbocharged engine!), such stability can be assumed in a first approximation to exist at least in an engine at operating temperature.
In a fuel injection system of the initially mentioned type (see Swiss Pat. No. 210 264, of Gebruder Sulzer, Aktiengesellschaft, dated during 1940), the pressure wave that passes through a short and moreover narrow-bore injection line to the injection valve causes the injection valve to open and, consequently, to effect pre-injection before the pressure wave propagating through a long and moreover wide-bore line reaches the injection valve. By means of an adjustable throttling valve (restrictor screw), the amount of fuel flowing through the short line is adjusted, for instance as a function of an operational parameter (load or speed).
This prior-art system suffers from a number of disadvantages. On the one hand, as a result of the narrow short line, the pressure at the outlet of the injection pump, which is a function of the load and speed of the engine, determines the amount of fuel for pre-injection and its time pattern. On the other hand, there is the risk in the partial-load range that the pressure surge transmitted through the short line to the injector is insufficient to open the nozzle needle. In this case, which is especially important with respect to noise, it is then necessary to shut off the short line, so that operation has to be continued without pre-injection and with the disadvantage of the hydraulic accumulator effect in the long line (which causes injection to be delayed during the build-up of the pressure in the injection line until eventually the injection valve opens). Moreover, there is no uncoupling of the pre-injection pressure wave relative to the injector end of the longer main injection line. This tends to result in undesirable pronounced variations of the line pressures in time and interference with the process of fuel injection into the combustion chamber (especially at partial load). The latter causes losses in the volume of pre-injected fuel because a pressure-reducing effect originates from the injector end of the large-bore main injection line.
It is therefore an object of the present invention, in a fuel injection system of the aforementioned general type, to provide accurate setting of the amount of fuel for pre-injection and the time interval relative to the main injection or, respectively, the points of commencement of the two fuel injections, and to make the operation practically independent of the level and variation in time of the pump pressure, i.e. ultimately independent of the speed and load of the engine.